An electronic control unit used in a vehicle for controlling an internal-combustion engine typically obtains physical quantities from an environment in which the vehicle travels, for a control of a drive of the internal-combustion engine. Humidity is one of such quantities. Since the internal-combustion engine mixes outside air taken in from the environment and fuel for the combustion, the electronic control unit needs to control the combustion based on the humidity of the outside air for an appropriate drive of the engine. Further, the electronic control unit also needs to refer to inside air humidity, i.e., humidity in a vehicle compartment (or in a passenger compartment), for controlling a degree of dehumidification performed by an evaporator when cooling the vehicle compartment by an air-conditioning device. Note that a vehicle compartment may also be designated as a passenger compartment in the following.
As described above, the electronic control unit obtains humidity information from an outside air humidity sensor and from an inside air humidity sensor disposed outside and inside of the vehicle, for controlling a drive of the internal-combustion engine and/or the evaporator. Therefore, failure of the humidity sensor may deteriorate the comfort in the vehicle, e.g., driver's comfort during the driving of the vehicle. In view of such a situation, various methods are proposed for detecting the failure of the humidity sensor. For example, a failure detection method for detecting the failure of the humidity sensor disclosed in a patent document 1, U.S. Pat. No. 7,654,253, is a method that detects a failure or degradation of the humidity sensor disposed outside of the vehicle, i.e., outside of the vehicle compartment, more practically. In such method, humidity obtained from the humidity sensor disposed outside of the vehicle compartment is compared with humidity obtained from the humidity sensor inside the vehicle compartment after a lapse of preset soak time after switching OFF of an ignition switch, and, based on a difference between two humidity values being greater than a preset value, it is determined that the humidity sensor is suffering from a functional degradation.
However, in the method disclosed in the patent document 1, it is unclear whether the length of the preset soak time is long enough against a variation of the humidity values obtained from the outside/inside air humidity sensors. That is, failure detection accuracy of such method may not be necessarily high. Further, if a sufficiently-long soak time is set, the failure detection accuracy may be improved, with a trade-off of a decrease or loss of the number of failure detection opportunities. That is, when the number of failure detection opportunities decreases, the detection of failure may be delayed.
More practically, when the number of failure detection opportunities decreases, the following drawbacks may be experienced. That is, the humidity value obtained from the inside air humidity sensor is normally expected to be different from the humidity value obtained from the outside air humidity sensor, due to either the dehumidification or humidification operation of the air-conditioning device. The soak time should basically be set to a length of time that is required for, i.e., sufficient for, the humidity value from the inside air humidity sensor to substantially return to the humidity value from the outside air humidity sensor after switching OFF the ignition switch. For the improvement of the failure detection accuracy for detecting the failure of the humidity sensor, the soak time may need to be set to a length of time based on the worst case, i.e., the maximum humidity value difference between the outside air humidity and the inside air humidity. In addition, the vehicle may be used in very diverse environments with varying humidity levels and the required soak time for covering all the various environments may need to be very long. Thus, a very long soak time may be too long for a vehicle used in a normal or average environment, i.e., decreasing the number of failure detection opportunities too much, and may delay the detection of a failure of the humidity sensor in the vehicle